1. Field of the Invention
The present invention relates to an AB class rail-to-rail operational amplifier.
2. Description of the Related Art
An operational amplifier has a finite voltage gain, finite input impedance and an output impedance larger than zero.
In general, the operational amplifier is required to have a high voltage gain and to maintain the high voltage gain over a high frequency. In general, the voltage gain of the operational amplifier decreases as the frequency increases.
The operational amplifier is also required to have a high input impedance and a low output impedance.
A complimentary metal oxide semiconductor (CMOS) operational amplifier having a rail-to-rail input stage and a rail-to-rail output stage is disclosed in U.S. Pat. No. 5,311,145. According to the CMOS operational amplifier of U.S. Pat. No. 5,311,145, a floating current source is connected to a summing circuit to provide current to the summing circuit, and a bias current is provided to output transistors of a class AB output stage.
According to the CMOS operational amplifier of U.S. Pat. No. 5,311,145, the area of the operational amplifier may increase because the floating current source has a plurality of transistors. In addition, the plurality of transistors is disposed along a signal path. According to frequency domain characteristics of the voltage gain, the voltage gain has a plurality of poles due to the transistors and wirings disposed adjacent to the signal path, so that the frequency characteristics of the operational amplifier may be degraded.
According to the CMOS operational amplifier of U.S. Pat. No. 5,311,145, two bias transistors QD1 and QD2 are coupled to each other in parallel, an output voltage of a diode-coupled circuit is applied to a gate electrode of each of bias control transistors. The CMOS operational amplifier of U.S. Pat. No. 5,311,145 may not be controlled by an external control signal, and a gate voltage of the output stage varies in accordance with various parameters that are determined in a manufacturing process of the operational amplifier, and thus static bias current of the output stage may depend on the manufacturing process of the operational amplifier.
Accordingly, the frequency characteristics and phase characteristics of the operational amplifier may be degraded due to the transistors and the wirings disposed adjacent to the signal path.